package main
import (
	"errors"
	"fmt"
)

func f1(arg int) (int,error) {
	if arg == 42 {
		return -1, errors.New("can't work with 42")
	}
	return arg + 3 , nil
}

type argError struct {
	arg int
	prob string
}

func (e *argError) Error() string {
	return fmt.Sprintf(" %d- %s",e.arg,e.prob)
}

func f2(arg int) (int , error) {
	if arg == 42 {
		return -1, &argError{arg,"can't work with it"}
	}
	return arg + 3, nil
}

func main() {
	for _, i := range []int{7,42} {
		if r, e := f1(i); e != nil {
			fmt.Println("f1 failed:",e)
		} else {
			fmt.Println("f1 worked:",r)
		}
	}
	for _,i := range []int{7,42} {
		if r,e := f2(i); e != nil {
			fmt.Println("f2 failed:",e)
		} else {
			fmt.Println("f2 worked:",r)
		}
	}

	_,e := f2(42)
	if ae,  ok := e.(*argError); ok {
		fmt.Println(ae.arg)
		fmt.Println(ae.prob)
	}
}
